1. Field of the Invention
The present invention relates to an image heating device that is suitable as a fixing device for fixing an unfixed toner image by heating a conductive belt directly or indirectly via a metal roller utilizing electromagnetic induction; an image forming apparatus, such as an electrophotographical apparatus or an electrostatic recording apparatus, using such an image heating device; an image copying machine using such an image forming apparatus; and a method for controlling temperature applicable to such an image heating device, an image forming apparatus, and an image copying machine.
2. Related Background Art
As image heating devices typically used for fixing devices, contact-heating type image heating devices such as roller-heating type devices and belt-heating type devices generally have been used.
In recent years, due to the demand for shorter warm-up time and reduced energy consumption, electromagnetic induction heating, by which rapid heating and high efficiency heating are likely to be attained, are attracting great attention. In the belt-heating type image heating devices, to shorten the warm-up time, a conductive belt having a smaller thermal capacity is used. A high-frequency current is applied to a magnetization coil to generate a high-frequency magnetic field, which causes an induced eddy current to be generated in the conductive belt, thereby causing Joule heat to be generated in the conductive belt itself. An unfixed toner image formed on a recording medium (paper, an OHP film, etc.) can be fixed after passing through a nip portion formed between a fixing roller and a pressure roller, which are pressed against with each other via the conductive belt that generates heat.
On the other hand, in the roller-heating type image heating devices, to shorten the warm-up time, a metal roller having a smaller thickness is used. A high-frequency current is applied to a magnetization coil to generate a high-frequency magnetic field, which causes an induced eddy current to be generated in the metal roller, thereby causing Joule heat to be generated in the metal roller. An unfixed toner image formed on a recording medium (paper, an OHP film, etc.) can be fixed after passing through a nip portion formed between the metal roller and the opposing pressure roller or between a fixing roller, to which heat conducted from the metal roller is transferred via a heat-resistant resin belt, and the opposing pressure roller.
In belt-type image heating devices (devices using a conductive belt or resin belt), a conductive belt having a small thermal capacity is heated through electromagnetic induction (direct heating of the belt), or a metal roller is heated through electromagnetic induction and the heat generated by the roller is conducted to a resin belt having a small thermal capacity (indirect heating of the belt). Thus, although the belt itself can be heated rapidly, a pressure roller having a large thermal capacity is heated slowly. Accordingly, in an early stage of the device operation, the temperature of the pressure roller is not sufficiently high while the belt already has reached a fixing temperature. Furthermore, if an intermittent printing operation is carried out continuously, the temperature of the pressure roller rises, and consequently, temperature fluctuations of the pressure roller become large. As a result, a toner image previously fixed and a toner image later fixed have a difference in gloss, or worse, fixing defects occur.
To solve such problems, in a conventional image heating device, it is necessary to provide a temperature sensor for detecting a temperature of a pressure roller in addition to a temperature sensor for detecting a temperature of the belt, so that the temperature of the pressure roller is taken into consideration when a fixing temperature is set. This configuration is intended to control an amount of heat generated by a heat-generating member according to the temperature of the belt and the temperature of the pressure roller detected by the foregoing temperature sensors so that the amount of heat applied to a recording medium at a portion where the belt and the pressure roller are pressed against each other is maintained at a predetermined reference level (see, for example, JP 6(1994)-149102 A).
However, it is not a preferable solution to provide an extra temperature sensor to detect a temperature of a pressure roller that serves for pressing a toner image onto the recording medium, that does not contribute directly to the heating of the recording medium, and that absorbs heat from the belt, since this causes an increase in the cost.
Furthermore, in the case where a temperature sensor is provided for the pressure roller, the sensor has to be placed within a range of a sheet width since a significant temperature fluctuation due to the passage of a sheet occurs with the pressure roller, but a surface of the pressure roller could be scarred by the temperature sensor, which in a double-sided printing operation might cause a scar in an image on a reverse side of a sheet. This is a significant problem, particularly in the case where a color toner image is to be fixed, since in such a case a pressure roller is required to have the same releasing property as that of the fixing roller and hence the pressure roller has a hard surface made of fluorocarbon resin, etc., in many cases.
In a type in which a metal roller is heated and the heat is conveyed by a resin belt, a rotating operation of the metal roller is generally started after the metal roller is heated up to a predetermined temperature, so as to shorten warm-up time. However, since the metal roller can be heated rapidly according to the electromagnetic induction heating, if the metal roller at rest is heated in the image heating device with a small thermal capacity, an abrupt temperature rise may occur partially. This may result in deterioration of the resin belt, an elastic material provided on the resin belt, and the like.
Especially in an image heating device performing heating with a metal roller and a resin belt looped around the roller, the temperature of the metal roller being made too high by the rapid heating results in a permanent deformation of the belt due to wrapping in accordance with the curvature of the roller. It is to be noted here that this problem seldom occurs in the case of a conductive belt and never occurs in a configuration in which a straight portion of the belt is heated. This problem occurs significantly only in a configuration in which a metal roller is heated and the heat from the roller is conveyed by the resin belt.
Furthermore, from the viewpoint of saving energy, it is preferable that a heat-generating member (conductive belt or metal roller) in an image heating device is heated only when the device is used. Image heating devices of the heat roller type generally include a heat-generating member at a nip portion. However, in image heating devices of the belt type, a heat-generating member is away from a nip portion, that is, a heat-generating portion of the conductive belt is away from a nip portion, or a metal roller that a resin belt is looped around is away from a nip portion, resulting in a time lag (thermal gradient) between a temperature change in the heat-generating portion of the conductive belt or in the heated portion of the resin belt, and a temperature change in the nip portion.
Furthermore, in order to respond to a print request from the user promptly, it is necessary to carry out preheating even during a stand-by period. However, in order to overcome the problem such as the wrapping or deterioration of the belt, which occurs when the belt is heated in a static state since the belt is heated rapidly to an extraordinarily high temperature, and to shorten as much as possible a time lag of a temperature change in the nip portion from a temperature change in the heat-generating portion of the conductive belt or the heated portion of the resin belt, the rotation of the conductive belt or the metal roller has to be continued even during the stand-by period. This is not preferable from the viewpoint of the energy saving or the suppression of noise caused by the rotation of the belt.